After a little more digging, it appears there was a Notebook that was shared with me in 2015. I killed all the notes and chose "Leave Notebook" from the notebook's ... menu. Unfortunately all the zombie tags are still there, unable to be deleted.
One thing that's interesting is that these only show up in the desktop and mobile apps, but not on the web.
I wanted to start using tags in Evernote, but with over 20 zombies, its' going to make it pretty hard.
Is there any kind of data check/cleanup tool? Maybe export everything then start over and re-import? I have over 1300 notes, so that's a little sacary as well. Or maybe deleting re-installing the apps?